It is well known in the art to pelletize beneficiated ore and subject the so-called green pellets to a heat-hardening process that enables them to withstand breakage during shipment and crushing in the stock column of a blast furnace. The most commonly employed method of heat-hardening is that of disposing the pellets on a pelletizing strand having a succession of pallets that carry the pellets under a hood or tunnel-like enclosure progressively through drying, preheating, firing, after-firing and cooling zones. A typical travelling grate apparatus for effecting heat-hardening of green pellets is disclosed, for example, in U.S. Pat. No. 3,172,754, Briggs, et al., assigned to the assignee of this application.
The initial step of drying is carried out at moderate temperatures to permit moisture in the pellets to escape gradually, for at high temperatures the moisture is converted to steam too rapidly, causing disruption of the pellets. After drying, the pellets are fired to a temperature sufficient to harden them but not at such a temperature to cause the pellets to fuse. Generally, temperatures in the range of 2300.degree.F. to 2500.degree.F. are employed, the heat being supplied from fuel burners located at spaced intervals along the walls of the firing hood. The burners are horizontally disposed above the pellet bed and preheated, recuperated air derived from the cooling zone is supplied to the individual burners from a central, longitudinally extending conduit through downcomer pipes, the heated combustion gases being drawn down through the pellet bed via windboxes disposed under the travelling grate.
When using highly radiant fuel, such as oil or powdered coal, it was found that the radiant heat from the luminous flame caused overheating of the pellets at the top of the bed where they were directly exposed to the flame, resulting in fusion of the top layer of pellets. To overcome this, a laterally extending refractory tunnel was built out from each burner port in the sidewall of the firing hood with the burner located at the outer end of the tunnel, the tunnel being of such length that most of the combustion space was provided within the tunnel, thus shielding the pellets from direct exposure to the radiant heat from the burners. This construction, however, is disadvantageous from the standpoints of initial cost and added maintenance expense since the refractory lining of the tunnels require regular replacement and repair due to the high destructive thermal conditions to which they are subjected.
Another proposed solution to this problem is provided in U.S. Pat. No. 3,620,519, Forbes, assigned to the assignee of this application, which provides a secondary enclosure or tunnel disposed within the main enclosure, the burners in one embodiment being located outside of and above the secondary tunnel which shields the pellets in the top of the bed from direct exposure to the radiant heat of the burner flame. This construction is also disadvantageous from cost and maintenance standpoints.